A typical image scanning device captures data by irradiating the object being scanned and capturing image data with charge coupled devices (CCD) or the like. The CCDs convert light reflected from the object to an analog electrical signal. In a typical photocopier there are thousands of CCDs per line of a scanned page with each line being scanned in succession. The data from each CCD is amplified and converted to digital data by an analog-to-digital converter (A/D), which outputs “codes.”
The highest output of the CCDs, which corresponds to the highest output code of the A/D, is commonly referred to as the “white level.” If the CCDs are sensitive to light of a particular color (e.g., red, green, or blue), the term white level is still used to refer to the highest output of the CCDs. The lowest output of the CCDs is commonly referred to as the black level.
Typically, target values for the A/D to output for the white and black levels are established to allow a wide range of output codes in the A/D. For example, for a 10-bit A/D the target white level code may be 960, while the target black level code may be 32.
To calibrate the white code level, a white reference region may be scanned. For example, the white region is scanned to determine white level codes output by the A/D. Based on the white level codes and a target white level code, the white level is calibrated. An adjustment may be made to an amplifier to calibrate the white level based on the analysis. Typically, only a few scan lines of white reference data may be collected. Therefore, the calibration should quickly converge to the target white level, or at least very close thereto.
Moreover, the black level also needs to be calibrated to a target black level. It is important that the technique for white level calibration is compatible with the black level calibration. For example, typically the white level is calibrated by adjusting the amplifier gain, while the black level is calibrated by adjusted on offset to the same amplifier. Calibration of the white level should not negatively impact the ability of the black level calibration to properly converge to the target black level or vice versa.
Thus, there is a need to provide a white level calibration for an image scanning device. Further, the white level calibration should quickly and accurately converge to the target white level. Furthermore, it is desirable that the circuitry be compatible with circuitry and techniques for converging the black level to a target black level.